Preparation of bromates



Feb. 27, 1940. c. H. MARTIN Er Al. 2,191,574

PREPARATION 0F BROMATES Filed urch 9, 1937 I l Cooums 1 SALT 35 PR E FEED Sernms 10 l /Box TA N K M Lcmmuuous FLow CELL l 30 3@ WATER JACKE-r SALT To CENTRIFua/u.

zo 24' l 2" INVENTOR 16 2z 28 f en@ New@ ATTORNEYS Patented Feb. 27, 1940 PATENT OFFICE 2,191,574 PREPARATION F BROMATESv Clair H. Martin and Elbert C. Hardy, Manistee, Mich., assig'nors to Morton Salt Company, Chicago, lll., a corporation of Illinois Application March 9, 1937, Serial No. 129,871

3 Claims.

AThis invention relates to the production of bromates. f

It is the principal object of the invention to provide a simple, economical, and commercially desirable process for the production of bromates such as potassium bromate and other alkali metal bromate's.

It is a further object to provide a highly satisfactory electrolytic process for .the production'of l0 bromates in Vwhich thebromate ,is secured in a continuous manner and under controlled and uniform conditions.

It is a still further object to provide such a process for the production of bromates utilizing l5 a depolarizer Ysuch as a chromate, in which the bromate is obtained in. a uniform and highly pure condition, and substantially free of undesirable color resulting from the chromate.

Other-objects and advantages will be apparent from the description,` the accompanying drawing, and the appended claims.

In the drawing,

Fig. 1 is a diagrammatic view or flow sheet representative of the practicing of the present 2s invention;

Figs. 2 and 3 are a top plan and a vertical sectional .View respectively of an electrolytic cell which is suitable for use in the practicing of the invention.

V:Axo' In accordance `with the "present invention there is provided a process for the production of bromates, which provides for the securing of the bromates in a highly pure condition, of v'uniform characteristics, and substantiallyfree of objec V tionable color. 'Ihe process is carried out elec- =trolytically, and in a continuous manneigfthe' conditions4 of the electrolytic reaction being at all times under denite control to provide forsecuring a product having the characteristics dey Lio-sired. The process is highly economical in its utilization of .materials and electric current and provides for the ready removal and recovery of a substantially pure bromate product ina simple and easy manner, avoiding the dimculties in `m-recovery of the product which result 'in the carrying out of a batch process.

Referring to Fig. 1 of the drawing which discloses a typical iiow sheet with apparatus suitable for carrying out the invention being illus- .5otrated diagrammatically thereon, .there is shown at I D a feed box through which the solution is supplied to the electrolytic cell, indicated diagrammatically at il. The solution or feed liquor as supplied to the cell is a solution of the bromide 55' from which it is vdesired to forma corresponding bromate. While not so restricted, the invention has been developed more particularly for the production of potassium bromate, and the following description will be presented using potassium as an example of an alkalimetal bromate 5v produced through the practicing of the invention.

'I'his feed liquor is preferablyV supplied in a solution of controlled concentration such for example as approximately a 25% solution of 10 potassium bromide. Suitable results are secured using a lower concentration, as itis found that in the continuous process of this invention lower concentrations, for example down to 5.0%, or even lower, produce entirely satisfactory results 15 in operation and the maintenance of the higher concentration ranges which are important in the operation of a batch process are not found to be necessary in the carrying out of the present invention. -The feed liquor also preferably con- 20 tains in solution a depolarizing agent, preferably an alkali metal chromate corresponding to the bromate which is produced. For example a solution containing .5 gram of potassium chromate per liter as a depolarizer has been used 25 with satisfactory results. A substantiallyv uniform concentration condition in the feed liquor supplied to the cell is preferably maintained by the controlled addition of the bromide solution with such addition of the chromate as required, $0. to bring the concentration of the bromide up to the desired range.

The temperatureV at which the feed liquor enters the electrolytic cell is important in connection with the solution temperatures main- 35 tained throughout the entire continuous cycle, in order that through the use of a controlled temperature condition at all points'in the cycle propersolubility characteristics will be maintained, in order to secure precipitation of con- 40 stituents in certain phases of the process and to avoid 'such precipitation where not desired.

f Ordinarily'it is preferred that the temperature .of the entering feed liquor be substantially that the cathode.' The-ipo hydroxide'th 2 ismade thereto as showing a preferred embodiment of cell structure'for this purpose.

The cell is generally rectangular in shape, and of elongated form, the side walls I5 and base It being formed of clear, close-grained hard wood having an overlying coating of Bakelite or baked on the surface thereof, to provide an inert surface in contact with the liquor. A series of anode members I1 are electrically connected with a positive bus bar I8 by suitable connectors I8, the bus bar extending along one side of the cell, and providing electrical connection and support for the spaced series of anode plates. Preferably these plates are formed of graphite although other suitable materials may also be used. Alternate plates are recessed into the It, as shown at 20, such plates terminating below the top of the cell. to leave a passage 2ly thereabove, the other group of plates alternating therewith being supported above the bottom of the cell to leave a lower passageway 22. lThis alternating relation of the anodes thus effects the homing of the liquor, requiring it to successively flow vertically upward and downward through 26, its passage through the cellto thereby secure substantial mechanical reaction of the liquor, re-

sulting in the bringlngspf the body of liquor in more intimate contact with the series of cathodes.

The cathodes are shown at 24, alternating with so each of the anodes I1 and supported from the bus bar by connectors 2l, the ends of which are clamped upon the end walls i! .of the cell, by means of clamping structure 2B, thereby supporting the bus bar 26 in elevated position along 86 one side of the cell. The cathodes are preferably also formed of graphite, andare of substantially less area than the corresponding anodes;

. for example in the structure shown, the .total area of the cathodes is approximately half the 40 area of the anodes.'l The cathodes are pomtioned substantially centrally located of the cell. and ex-` tend into contact with the base thereof, thereby requiring the liquid how to traverse the cathode transversely of the cell. The cathode thus provides an additional bailling action, requiring passage of the liquor in a direction substantially transverse to the baming action aorded by the anodes, the combined electrodes thereby securing a highly effective baming action. resulting in bringing the entire body of feed liquor quickly and substantially uniformly into reactive contact 'with both sets of electrodes.

At the discharge end of the cell there are provided a pair ofdischarge openings 21 through the base It, the openings being provided with'a glass tube 2l, extending therethrough, to permit l v.and being plated out as a metal thereon, the ,..bromine going to the anode. V,However as soon as the potassium is formedrassuch it immediately reacts -withthezwaten hydrox me with the evolution hydrosensagas at sasl with che: tramite-r temperature rises above this pointthe bromine "proper results without entailing the vdifficulties found that where the solution from the batch cell the production of potassium hypobromite and potassium bromide. The potassium hypobromite by a self-oxidation process which' takes place under suitably controlled conditions converts itself .into

potassium bromate in accordance with the fol- 6 lowing equations, showing the1 total quantities of reactive ingredients required. ,l

It is highly important in the carrying out of these reactions to secure the bromate, that the` proper conditions are maintainl and controlled within the cell. Thus it is important that the temperature of the cell be controlled, and preferably maintained below the boiling point of bromine which is of the order of C. If .the

tends to pass out of solution, and is lost from the process. This is undesirablenot only because ofl the loss ofthe bromine itself, butl because such loss results in the unbalancing of the reaction, certain of the potassium hydroxide being unable to react with bromine, and therebyl building up and increasing the alkalinity ofthe liquor. Increase in alkalinity of the liquor tends to result in stabilization of the hypobromite, thereby retarding-the self-oxidation process and tending to prevent the conversion of the hypobromite into bromate. This control of alkalinity is also' bes lieved to have an important eect upon the subsequent processing and color condition in the bromate crystals as will be discussed morel in gl detail hereafter. 1

In the batch operation it was ordinarily necessary to operate the cell under a higher alkalinity condition in order tosecure the diffusion and interaction as between the potassium hydroxide and the bromine. But in the practicing -of the invention of this application a lower concentration of potassium hydroxide and bromine are preferably used, with entirely satisfactory r`esults because of the mechanical intermixture action taking place in this cell as a result of the passage of liquor therethrough.

Thus during its passagev through the lcell \the liquor is continually brought intoA intimate contact with both anode and cathode surfaces, so that the products of the electrolysis are brought into intimate reactive contact with each other even when present at low concentrations. This is highly desirable since it provides for securing resulting from higher concentrations of bromine.' and of caustic. While -it is difficult to secure actual 'pH value measurements because'of the strongly oxidizing character of the solutions, in actual operations on a plant scale it has been is strongly alkaline to litmus, that from the continuous cell of this invention is not strongly alkaline.

It is also believed that --the proper control of the pH condition during the Aelectrolytic operation has an important eifect in regard tothe color of the bromate yas produced. The color may be imparted-to the bromate originally by the chromate used as agdepolarlzer, usually resulting in 7b',

coloring the bromate in the same manner as a solution ofthe chromate itself. When the bromate is formed in the batch' process, with a high'- er alkalinity condition, it yseems that the chro- 7./ 'Tmate is incorporated into the bromate' crystals 7' or cake as formed, the combination apparently being some complex molecular or physical interrelationship which makes it highly difilcult to separate the chromate from the bromate. In .certain cases where bromate crystals have been produced under such high alkalinity conditions and have an objectionable color, it is found vthat even though such crystals are dissolved and recrystallized a number of times the color still remains, gradually fading out with such successive crystallizing operation'. The Hochwalt and Waliuszis Patent 1,919,721, assigned to the same assignee as the present invention, discloses certain inventions providing for the production of a decolorized bromate product from the batch process.

In accordance with the present invention however the bromate as initially produced, under the controlled conditions described, is substantially free of color, the chromate apparently not being attached or incorporated with the bromate in the same manner as occurs in the batch` process. The suspension of the bromate crystals may have a colored appearance because of the presence of the chromate in solution in the liquid; however this colored solution can be readily washedout, substantially completely freeing the bromate crystals of undesired color without the necessity of dissolving and recrystallilzing the bromate itself. And it is believed that this production of the bromate crystals in a different form, and substantially free of color, arises 'as a result of the predetermined controlled pH value conditions existing during formation of the bromatejin the electrolytic cell.

A preferred temperature condition for the operation of the cell has been found to be of the order of 45 C. to about 50 C. As explained above, higher temperatures tend to result in un-y desirable loss of bromine, and at temperatures materially below this range the resistance of the cell is found to increase somewhat, resulting in lower operating eiliciencies. Also it is important to provide for inhibiting the reducing action of the hydrogen as produced at the cathode, the hydrogen havingv a tendency if not removed 'to reduce the hypobromlte. For this reason it iS preferred to make the area of the cathodes substantially less than the area of the anodes to thereby increase the tendency of the hydrogen to bubble up lthrough the liquid and escape as a gas. Heat also tends to assist in this escape of the hydrogen and accordingly it is preferred to operate the cell generally'within the temperature range indicated labove. The

action of the depolarizer facilitates the escape l of the hydrogen and also appears to function in the nature of a catalyst, tending to vprevent an objectionable reduction action taking place at the cathode.

The rate of flow through the cell is preferably regulated to produce the desired outlet temperature in the liquor. In an experimental operation, the rate of flow was 8 gallons per minute,A

with a current, of 1000 amperes. The current density may be higher in the present process,-

than in a batch process vbecause as already explained it is not necessary to rely upon diffusion of the products of electrolysis', such diffusion actionbeing greatly augmented by the mechanical movement. of the liquor. been found entirely practicable to use higher current densities, 'while maintaining good current eiliciencies.A As the 'current density increases the overvoltage also increases, and there is therefore a practical upper limit which should Arid it has.

not be exceeded for economical reasons. The reduction in the relative size of the cathode tends to increase the cathode drop or overvoltage, and

thus while a much smaller cathode area facilitates the depolarizing action of the cell, Ait is not advisable that the cathode be so small as to result in vexcessive overvoltage. As above indicated, the preferred ratio of cathode to anode area for practical commercial operations has been found to be approximately 1 to 2.

Satisfactory results have been secured with current densities of the order of 7 amperes per square decimeter of anode surface with a voltage of about volts. The voltage is increased with higher current densities, for example at a current density of 10 amperes per square decimeter a voltage of about '7% volts is desired, and at a current density of amperes per square decimeter, the voltage desired is about 12 volts.

As will be apparent from the reaction equations set forth above, the reactions taking placein the electrolytic cell result in the production' 50 C. a saturated solution of potassium bromate contains approximately 14.9% bromate. The temperature of the liquor as it leaves `the cell, as already stated, is preferably controlled to be of the order of to 50 C. And at that temperature the solution is far from being saturated either With the brolnide or vthe bromate, so that both salts Will readily remain in solution.

As the treated liquor issues from the discharge openings in the cell, it first passes through a lter indicated generally at 30, which may be of any desired type, for the purpose of filtering out any insoluble foreign particles which are suspended in the liquid, such for example as small particles of graphite which have a tend- -ency to ilake oif from the electrodes. The iii itating tank 33.-. This tank is provided with av water jacket or similar means to effect reduction in the temperatureY of the liquor, and is further preferably provided with an agitating member 34 for maintsining the precipitated particles in suspension in the liquid and avoiding settling at the bottom of the tank.,

- In the tank the liquor is cooled to a suitable temperature to effect the precipitation of bromate from the solution, the bromate precipitating out as crystals which are held in suspension in the liquor and are prevented from settling in a hard cake at thev Abottom of the tank as a result of the agitation. Agitation also serves to facilitate heat transfer from the liquor tothe cooling medium through movement of the hot liquor, and further is effective in keeping the crystals in suspension where they act as seed crystals, thus avoiding undesirable supersaturation. Itis desired to 'so control the temperature condition throughout the. process as to require n bei. am t0. auch suse and by 4 l cooling tank. This is accom by so reg q ,Y ulating the temperature that no ifurther temperature drop takes place in the following thecooling of the liquor within the tank. o For this purpose the liquor may be cooled to room temperature, or preferably lto a temperature slightly below room temperature so that any subsequent change of temperature in the liquor will be an increase in temperature rather. I than avdrop in temperature. v o

'I'he bromate content of the liquor as it is supplied to the cooling tank is such that the saturation point of the liquor for the bromate will be reached before the temperature is cooled to the room temperature condition as described, thereby resulting in the precipitation of bromate from the solution. Even though there is a larger molecular weight of bromide present, because of f the much greater solubility of the bromide,` its -saturation point is not reached even at such lowered temperature condition, and no bromide is precipitated out.- Thus there results a highly eifective and quite pure separation, of bromate, free of bromide, with all of the bromide remaining in solution for subsequent reuse in the cyclic process. ,o

In the batch process. the bromate was usually precipitated out in the bottom oi' the cell, in the form of a hard cake, which was diiilcultto take out of the cell, requiring a separate removal op eration of considerable diiilculty. In accordance with the present invention, and through the use and maintenance of controlled temperature and solubility conditions throughout lthe process, the -lprecipltation of the bromate takes place\ in a vsuspended condition, so that it is subject to being y recovered from suspension as ilnely divided particles or crystals, in a much easier form than the hard cake produced in the prior art. method. Following the precipitation, the suspension is run into a settling tank il where the salt is snowed to seti-.1e m the bottom of the tank, being from time to time. This 'the wash water containing certain of the Y bromides in solution being returned to the system as make-up water so that complete recovery of the bromides, and any dissolved bromate, is se- .toured The washed bromate crystals marthen .be subjected to the action of acentrifuge to dry the separated salt which is then available for use 'I'he supernatent liquor is ilowed over the tank after the settling `operation and is returnedy tothe feed box `it' to be used subsequently in the process. Preferably the make-up ingredients which. comprise freshv water andxpot'aasimri bromide. are also added at the ieed'box. elexample the Irequired amount of bromide in order 5to maintain'ta desired conoentrationof-bromide in the liquor may be added from time to time' in solution form tothe feed box, vfromthe'centrifuge with additional n being used fox-'disclosing the bromide: For the bromide may be placed in solid form .beneath the inlet to, iced so that Athe. liquor flowing into the feed v fresh salt and carry it into theceilin solution o for subsequent use. All of the ,tanks and cells are preferably opera 'd in the Afull condition as by means of an overilow so that one level gage, preferably positioned Ionthe settling tank, will accurately measure the total liquid -in the system.

ent

'menen and pH value conditions,

'alkallnies in the electrolytic cell, with the Utt- .faciiitating the recovery -irom time to time of the concentretionof the bromide in the liquor, the necessary addition of waterandfresh bromidemaybemadetosecure Y. and maintain the desired concentration condi- Thus the present invention provides a highly satisfactory process for the production o! bromates, providing for economical and efllcient production with a minimum loss of reactive constituents, and a minimum requirement in quantity of electrical energy. Through the proper maintenance and control of desired temperature A unn loss of bromine is avoided, and the building up of high tendant obiectionable results thereupon, are eectively avoided, The invention further provides for the production of bromates which are substantially free of color, and'do not require separate pr to purify and decolorise the product. And further the bromate as produced is substantially free of bromides and is obtained in desired crystals in finely divided form greatly of the product from the u While the process herein described constitutesa preferred embodiment of the invention, it is to bel understood that the invention is not limited to this precise process, and that changes maybe made therein without departing from the scope of the invention which is defined in the ap-A pended claims.

what is claimed is: v

1. A continuous electrolytic process for the production of alkali metal bromates from an-aqueous solution containing the alkali metal bromide ,and an alkali metal chromate as a depolarizer with the production oi' the alkali metal bromate in a condition substantially free of discolorationresuiting from such chromate which comprises dowing continuously through an eleotrolytic cell containing a plurality of spaced anode and cathode electrodes a stream of said bromide solution, g an electric current through the cell to cause continuous formation of reactive bromine at the anodes and the .alkali metal hydroxide at the cathodes. simultaneously diffusing the formed bromine and hydroxide in-substantially the quantities as formed into the solution in its u.: e through the cell" by causing movement. of substantially the entire body ofthe liquid within the cell into intimate and absorbing contact with the successive anodes and cathodes at e rate controlled to absorb and remove` from each electrode the entire quantity of materials formed thereon and to diusev the materials from the several electrodes into and through the lsolution into intimate reactive contact with each other to form hypobromite and resulting in solution with concomitant temperature rise of the solution, controlling the temperature ofithesolution so that it reaches .a maximum value' during its through the ,cell which provides for maintaining the btomltefln solution lli'd'limlted; to be not in excess of that at winch the bromine remains in solution phase and availablefor said reactive contact in substantially originato formed `-quantity and approximately .45`..C- to 50 C.

ereby maintaining low alkalinity"in the solution. and' avoiding occlusion of thefchromate upon the bromate, and withdrawing'the toca'use precipitationv of crystals of bromate substantially clear and free of contaminationwith lll' soldtion'from the cell and cooling the solution 2. A continuous electrolytic process for thel production of alkali metal bromates from an aqueous solution containing the alkali metal bromide and an alkali metal chromate as a depolarizer with the production of the alkali metal bromate in a condition substantially free of discoloration resulting from such chromate which comprises ilowing continuously through an electrolytic cell containing a. plurality of spaced anode and cathode electrodes of carbon a stream of said bromide solution of a concentration range of approximately 5% to 25%, passing an electric current through the cell to cause continuous Iormation of reactive bromine at the anodes and the alkali metal hydroxide at the cathodes, simultaneously diffusing the formed bromine and hydroxide in substantially the quantities as formed into the solution in its passage through the cell by causing movement of substantially the entire body of the liquid within the cell into intimate and absorbing contact with the successive anodes and cathodes at a rate controlled to absorb and remove from each electrode the entire quantity of materials formed thereon and to diffuse the materials from the several electrodes into and through the solution into intimate reactive contact with each other to form hypobromite and resulting bromate in solution with concomitant temperature rise of the solution, controlling the temperature of the solution so that it reaches a maximum value during its passage through the cell which provides for maintaining the bromate in solution and limited to be not in excess of that at which the bromine remains in solution phase and available for said reactive contact in substantially originally formed quantity and approximately 45 C. to 50 C. thereby maintaining low alkalinity in the solution and avoiding occlusion o'f the chromate upon the bromate,

and thereafter withdrawing the solution i'x'om the cell and cooling thesolution to cause precipitation of crystals of bromate substantially clear and free of contamination with occluded chromate in suspension in the liquor.

3. A continuous electrolytic process for the production of alkali metal bromates from an aqueous solution containing the alkali metal bromide and an alkali metal chromate as a depolarizer with the production of the alkali metal bromate in a condition substantially free of discoloration' passage through the cell by causing movement of substantially the entire body of the liquid within the cell into intimate and absorbing contact with the successive anodes and cathodes at a rate controlled to absorb and remove from each electrode the entire quantity of materials -Iormed thereon and to diffuse. the materials from the several electrodes into and through the solution into intimate reactive contact with each other to form hypobromite and resulting bromate in solution at a concentration in excess'or approximately 6.5% and less than 14.9%, with concomitant tern`4 perature rise of the solution, controlling the temperaturevof the solution so that it reaches a maximum value during its passage through the cell which provides for maintaining the bromate in solution and limited to be not in excess of that at which the bromine remains in solution phase and available for said reactive contact in substantially originally formed quantity and ap` proximately 45 C. to 50 C. thereby maintaining low alkalinity in the solution and avoiding occlusion of the chromate upon the bromate, and thereafter withdrawing the solution from the cell and cooling the solution to cause precipitation oi' crystals o! bromate substantially` clear and free of contamination with occluded chromate in suspension in the liquor.

CLAIR H. MARTIN. ELBERT C. HARDY. 

